Hydraulic directional control valves are used to direct the flow of oil in the hydraulic lines according to the working position taken by the valves. For example, a valve with four ports, with one port connected to the high oil pressure source, one port connected to the tank, and the other two ports connected to two sides of an actuator can assume three working position. In the first position, when the valve is not given any input signal, the valve takes the central position. If in this valve position the actuator is required to remain unmoving, the ports connected to the actuator should be closed. If the valve is supplied by an input signal to assume another working position, one of the actuator ports may be connected to the high oil pressure port while the other actuator port is connected to the tank, and the actuator consequently would move in one direction. When the valve receives another input signal to be in its third working position, the connections of the port are reversed and the actuator would move in the opposite position. The time required for the valve to move from one working position to the other is about one tenth of a second, and this time depends on whether the valve is a direct or pilot operated. In pilot operated valve, a pilot valve is used to actuate the main valve which controls the actuator to overcome the opposing forces that might restrict the motion of the main valve. The most widely used directional control valves are the spool valves that connect and disconnect the valve ports in different possible ways according to control requirements.
Hydraulic servo valves, and contrary to the directional control valves are characterized by the fact that their spools can assume infinite number of working position. This allows the servo valves to open partially, and allows the accurate control of the flow rate and/or the oil pressure, beside controlling the flow direction. The speed of response of the servo valve is much higher than that of the directional control valves with speed of response of about 0.01 seconds. This high accuracy and speed of response cannot be achieved without the high machining accuracy that results in a high production cost. Direct operated servo valves allow only low flow rates, and for the moderate and high flow rates pilot operated servo valves of high cost should be used.
A third type of valves with spools that can assume infinite number of working position, as in servo valves, but is of less accuracy, lower speed of response, and lower production cost is the proportional valves. A valve of the propositional type can replace several valves of the ON/OFF type.